How to implement such protection against a headless browser?

Question

JavaScript

Recently, there are sites that somehow manage to block tools like Puppeteer in the bud! But how do they do it is not clear?
For example - https://www.expertflyer.com
If you try to access this site, you will see how the connection is instantly disconnected. Nothing is even loaded into DevTools!!!
Well, i.e. okay, if it was some kind of JS check by the code on the client. Right there, everything is torn stupidly. How the hell?
The user-agent has of course been changed. All methods of bypassing the detection known to me are also applied, but as I say, nothing is even loaded, it is stupidly torn. How did they do it? Is it possible to somehow investigate what is still loaded on the client at the first request (from DevTools it all disappears after the break)

Answer 1

there are no
miracles in FF more resistant than Chrome to anti- Selenium troubles
, put a fidler, look carefully what's there, who's there,
if it really breaks without JS - it's about IP